1. Field of the Invention
The present invention generally relates to a method for adjusting the back focus and the gradient of a solid-state image detector, such as CCD (Charge Coupled Device), and more particularly to a novel position adjusting method for a solid-state image detector capable of speedily positioning the solid-state image detector at an optimum position in the image-forming optical system.
2. Prior Art
A conventional method for positioning a solid-state image detector at a desired position in the image-forming optical system will be explained with reference to FIGS. 29 through 32.
FIG. 29 is a schematic view showing a conventional system for positioning a solid-state image detector at a desired position in the image-forming optical system.
In FIG. 29, a two-dimensional resolution chart (hereinafter referred to as 2-D resolution chart) 1 is provided in front of a master lens 2. This 2-D resolution chart 1 is characterized in that an alternate black-and-white pattern, serving as an adjusting pattern 1a, is provided at each of four corners of the rectangular 2-D resolution chart 1, as shown in FIG. 30. Each adjusting pattern 1a is used for detecting the focus point of master lens 2.
More specifically, a light beam, emitted from a light source 15 placed at an obliquely outward position from 2-D resolution chart 1, reaches the surface of 2-D resolution chart 1 at a given incident angle, and then reflects from the surface of 2-D resolution chart 1 toward master lens 2. Thus, the reflection light image of 2-D resolution chart 1 reaches the master lens 2.
A color separation prism 3, which is used to separate an entered light into three color components, R, G and B, is disposed behind the master lens 2. For receiving each of these color components R, G and B, a total of three solid-state image detectors 4 are independently disposed around the color separation prism 3 in a close confronting relationship. Each solid-state image detector 4 is connected to a video signal processing circuit 5 which is further connected to a TV monitor 6.
Positioned behind each solid-state image detector 4 is a positioning mechanism 7 which moves or shifts the solid-state image detector 4 to the focus point of master lens 2. A correction drive circuit 8 is connected to each of positioning mechanisms 7 to control them. The correction drive circuit 8 is connected to a calculating circuit 9a, which is one of circuits in a VME system 9. In addition to this calculating circuit 9a, VME system 9 further comprises an image processing board 9b and a digital conversion circuit 9c.
The calculating circuit 9a has a function of calculating an integration (or accumulation) value with respect to the contrast. The image processing board 9b has a frame memory. The digital conversion circuit 9c has a function of converting a video signal received from the video signal processing circuit 5 into a digital signal.
An operation of the above-described conventional solid-state image detector positioning system will be explained hereinafter.
In FIG. 29, the image of 2-D resolution chart 1 is formed on the solid-state image detector 4 through master lens 2. The video signal of 2-D resolution chart 1, which is an electric signal generated from the solid-state image detector 4, is entered into the video signal processing circuit 5 and then displayed on the TV monitor 6. The output signal of video signal processing circuit 5 is also fed to the digital conversion circuit 9c in the VME system 9 where the entered video signal is converted into a digital signal.
The image processing board 9b receives the digital signal from the digital conversion circuit 9c and stores the digitized video data in the frame memory thereof. The calculating circuit 9a in the VME system 9 arithmetically obtains the "contrast integration value" based on the digital video data representing the alternate pattern of the 2-D resolution chart 1 which is stored in the frame memory of image processing board 9b.
The "contrast integration value" represents an integration or accumulation of all the luminance-differences between adjacent two pixels on the entire video image displayed on the TV monitor 6. Having a large contrast integration value is having a higher resolution in the output image. In other words, it is believed that the focus point of master lens 2 is accurately positioned on the solid-state image detector 4. Thus, by detecting the contrast integration value, it can be confirmed that the concerned solid-state image detector is surely positioned at a desirable position in the image-forming optical system.
Next, the positioning mechanism 7 slightly shifts the corresponding solid-state image detector 4 in the direction of the optical axis direction "Z". After finishing the slight shifting operation of the solid-state image detector 4, the above-described sequential operations are repeated again to arithmetically obtain a new contrast integration value at a resultant position of solid-state image detectors 4.
Furthermore, solid-state image detector 4 is slightly shifted in the direction of optical axis "Z". In response to each shift movement, VME system 9 receives the video signal of solid-state image detector 4 and calculates the contrast integration value, finally obtaining the characteristics of FIG. 32 showing contract integration value "F" in relation to solid-state image detector position "D" with respect to the optical axis "Z".
Referring to the characteristics of FIG. 32, it is known that the contrast integration value "F" becomes maximum at the position "P" where the master lens 2 focuses the image. Thus, the positioning mechanism 7 moves the solid-state image detector 4 to thus detected position "P".
In this manner, through the adjustment of the back focus each solid-state image detector 4 can be positioned at the predetermined position in the image-forming optical system.
Next, the conventional method for adjusting the gradient of each solid-state image detector 4 with respect to the image-forming optical system will be explained.
In the VME system 9, digital conversion circuit 9c receives the video signal of 2-D resolution chart 1 and converts it into a digital signal. The image processing board 9b stores the digitized video data in its frame memory. Thereafter, based on this digital video data, calculating circuit 9a arithmetically obtains the contrast integration value of each adjusting pattern 1a located at four corners of the rectangular 2-D resolution chart 1 shown in FIG. 30.
The arithmetically obtained value is then sent to correction drive circuit 8 which actuates the positioning mechanism 7 to slightly incline each solid-state image detector 4 with respect to the optical axis "Z" in the X-axis direction and Y-axis direction.
In response to each of such a slight inclination of solid-state image detector 4, VME system 9 arithmetically obtains a new contrast integration value of each adjusting pattern 1a located at four corners of the rectangular 2-D resolution chart 1. Obtained by repeating the above-described sequential operations is a specific gradient by which the contrast integration value is equalized at all four adjusting patterns 1 of the 2-D resolution chart 1. Then, the position or attitude of each solid-state image detector 4 is adjusted to meet the obtained specific gradient.
In this manner, through the adjustment of gradient each solid-state image detector 4 can be positioned at the predetermined position in the image-forming optical system.
As apparent from the foregoing description, the above-described conventional solid-state image detector positioning system adjusts the back focus of each solid-state image detector by shifting the solid-state image detector in the back-and-forth direction with respect to the optical axis so as to finally detect the focus point of the master lens. Furthermore, the above-described conventional solid-state image detector positioning system adjusts the gradient of each solid-state image detector based on the image displayed on the TV monitor in such a manner that the contrast integration values of all the adjusting patterns located at four corners of 2-D resolution chart can be equalized to the same value. Hence, the inclination of each solid-state image detector in the given image-forming optical system can be correctly adjusted.
However, according to the above-described conventional solid-state image detector positioning system, there was a problem that detection of the focus point of the master lens was troublesome. That is, this conventional method definitely requires to detect the specific position where the contrast integration value becomes maximum, using the image of the 2-D resolution chart obtained by each solid-state image detector. To detect the maximum value of the contrast integration value, it is necessary to shift the solid-state image detector in the back-and-forth direction of the optical axis and to arithmetically find out the maximum value of the contrast integration value by repetitively calculating the contrast integration value in response to each slight movement of the solid-state image detector. Accordingly, it took a long time to finish and was not possible to speed up the detection of the focus point of the master lens.
Furthermore, there was a problem that the detection of gradient of each solid-state image detector was complicated. To obtain an optimum gradient of each solid-state image detector in the image-forming optical system, it is definitely necessary to repetitively swing the solid-state image detector in the horizontal (i.e. X axis) and vertical (i.e. Y axis) directions until the contrast integration values of all the adjusting patterns located at four corners of the 2-D resolution chart are equalized. Hence, it was not possible to speed up the detection of the optimum gradient of each solid-state image detector.
Furthermore, there was a problem that at a certain time during the adjustment it was not possible to obtain the information regarding the present position of the concerned solid-state image detector with respect to the designated position, i.e. with respect to the focus point of the master lens.
Still further, there was a problem that relying on visual adjustment based on the camera image displayed on the monitor screen might fail to accurately detect the focus point of the master lens, although the camera image represents the adjusting pattern of 2-D resolution chart which is obtained from each solid-state image detector.
Furthermore, there was a problem in the image-forming optical system that the farther the incident light inclined from the optical axis the greater the image of light deviated or distorted from the position to be derived from the Newtonian formula of focusing of the lens. As a result, distortion aberration will be caused as shown in FIG. 33 wherein the shape of the substance does not completely coincide with the shape of the obtained image. Thus, providing the adjusting patterns at four corners of the resolution chart is subjected to the distortion aberration of the master lens, lowering the detecting accuracy in the detection of the focus point of the master lens.
Furthermore, there will be a problem in the detection of the focus point of the master lens that image magnification may be differentiated by the difference of distance from the master lens.
Yet further, there was the problem that illumination irregularity was caused due to difference of distance between each area on the resolution chart and the illumination light source when the illumination light was entered at a significant incident angle from the light source placed at an obliquely outward position.
The size of the resolution chart will be the problem to be solved when it is large.
Moreover, it will be necessary to adjust the back focus and the gradient if they are undesireably caused after the solid-state image detector is firmly fixed by adhesive material, because hardening of such adhesive material possibly causes thermal expansion and contraction stress.